(1) Field of the Invention
The present invention relates to a transmission timing adjusting device for a transmitting/receiving apparatus equipped with two transmitting/receiving means, one for actual use and the other for standby reserve, for carrying out two-way communications via an identical communication path, and more particularly, to a transmission timing adjusting device for a transmitting/receiving apparatus which constitutes an optical coupler-connected 2:n redundant system in a PON (passive optical network) and which carries out optical communications between a station equipment and each of subscriber equipments through a single optical fiber according to TCM (time compression multiplex) procedure.
(2) Description of the Related Art
In conventional transmitting apparatuses using TCM-TDMA (time division multiple access) procedure, signals transmitted from a plurality of subscriber equipment through respective optical fibers are multiplexed by a star coupler before they are sent to a station equipment. When the signals are multiplexed, array control is performed such that burst signals from the respective subscriber equipments do not superpose one upon another. This array control, however, cannot be proper if the burst signals are arrayed simply according to their burst lengths. Namely, since the lengths of optical fibers connecting the respective subscriber equipments to the star coupler usually differ from one another, the burst signals are subject to different transmission delays, depending upon the fiber lengths. Accordingly, the burst signals cannot be multiplexed at respective predetermined positions and may possibly be superpose one upon another.
To avoid this, the delay time of the burst signal from each subscriber equipment is measured beforehand at the station equipment, and the transmission timings at which the burst signals are transmitted from the respective subscriber equipments are adjusted based on the difference of the measured values, thereby correcting the multiplexing positions of the burst signals.
However, this correction method is effective only in the case where the station equipment involves no redundancy. Where the station equipment has redundancy and at the same time the length of an optical fiber connecting the star coupler to an active system which is currently in use differs from that of an optical fiber connecting the star coupler to a standby system which is for standby reserve, a problem arises in that up and down signals superpose one upon the other when the system is switched from the currently active system to the standby system, as described below. A means of solving the problem of superposition of up and down signals associated with a switching from the active system to the standby system is disclosed, for example, in Unexamined Japanese Patent Publication (KOKAI) No. 56-21436, though the apparatus disclosed is not an optical communication apparatus but a satellite communication apparatus. This satellite communication apparatus is a redundant communication apparatus of TDMA type, wherein a radio wave is always transmitted from the standby system to correct the multiplexing positions of burst signals so that system discontinuance may not occur when the system is switched from the active system to the standby system.
As shown in FIG. 8, in the case where a station equipment 101 is provided with a transmitting/receiving circuit 101a for actual use and another transmitting/receiving circuit 101b for standby reserve, the length of an optical fiber 103 connecting a star coupler 102 to the active transmitting/receiving circuit 101a usually differs from that of an optical fiber 104 connecting the star coupler 102 to the standby transmitting/receiving circuit 101b. The star coupler 102 is also connected to a plurality of subscriber equipments 105 to 107, each via an optical fiber, as mentioned above.
When the circuit used is switched from the active transmitting/receiving circuit 101a to the standby transmitting/receiving circuit 101b, it is necessary that the transmission timings for up signals (signals from the subscriber equipments to the station equipment) should be corrected. This will be explained with reference to FIG. 9 which illustrates the timings of signal transmission and reception between the station equipment and the individual subscriber equipments.
As illustrated, down signals S21 and S26 are transmitted from the active transmitting/receiving circuit 101a of the station equipment 101 to each of the subscriber equipments 105 to 107 at intervals of unit time. On the other hand, the subscriber equipments 105 to 107 transmit up signals (burst signals) S22 to S24 to the active transmitting/receiving circuit 101a of the station equipment 101 at respective predetermined timings based on the transmission timing of the down signal S21. These timings are determined in consideration of the transmission delay of a signal from a reference subscriber equipment to the active transmitting/receiving circuit 101a, and are corrected based on the difference in fiber length between the star coupler 102 and the respective subscriber equipments 105 to 107, as mentioned above. Accordingly, the up signals S22 to S24 can be arrayed at predetermined intervals between the down signals S21 and S26.
If, however, the optical fiber 104 connecting the star coupler 102 to the standby transmitting/receiving circuit 101b is, for example, longer than the optical fiber 103 connecting the star coupler 102 to the active transmitting/receiving circuit 101a, when the circuit used is switched from the active circuit 101a to the standby circuit 101b the times at which the up signals S22 to S24 reach the standby circuit 101b are delayed. Consequently, a situation could arise where the standby transmitting/receiving circuit 101b starts transmitting the down signal S26 before the last up signal S24 reaches the standby circuit 101b (indicated at S25 in FIG. 9).
To avoid such a situation, when the circuit is switched from the currently active transmitting/receiving circuit 101a to the standby transmitting/receiving circuit 101b, the active circuit 101a must be completely shut down once to suspend the data transfer between the station equipment 101 and the subscriber equipments 105 to 107, and transmission delays of signals from the respective subscriber equipments 105 to 107 to the standby circuit 101b must be measured in order to rearrange the multiplexing positions of the burst signals. The measurement of transmission delay is carried out for each of the subscriber equipments and thus consumes considerable time before recovery, and communications during the measurement are not feasible.